Compositions and methods for polishing (e.g., planarizing) the surface of a substrate are well known in the art. Polishing compositions (also known as polishing slurries) typically contain an abrasive material in an aqueous solution and are applied to a surface by contacting the surface with a polishing pad saturated with the polishing composition. Typical abrasive materials include metal oxide particles, such as silicon dioxide, cerium oxide, aluminum oxide, zirconium oxide, and tin oxide. U.S. Pat. No. 5,527,423, for example, describes a method for chemically-mechanically polishing (CMP) a metal layer by contacting the surface with a polishing composition comprising high purity fine metal oxide particles in an aqueous medium. The polishing composition is typically used in conjunction with a polishing pad (e.g., polishing cloth or disk). Suitable polishing pads are described in U.S. Pat. Nos. 6,062,968, 6,117,000, and 6,126,532, which disclose the use of sintered polyurethane polishing pads having an open-celled porous network, and U.S. Pat. No. 5,489,233, which discloses the use of solid polishing pads having a surface texture or pattern. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,958,794 discloses a fixed abrasive polishing pad.
Despite the advantages of polishing compositions comprising metal oxide particles, these polishing compositions often become colloidally unstable (i.e., the metal oxide particles coagulate and fall out of suspension) within certain pH ranges. For instance, polishing compositions comprising silica particles are known to be colloidally unstable at mildly acidic pH (e.g., pH of about 4-6). Such colloidal instability and the resulting precipitation of the metal oxide particles severely limit the effectiveness of these polishing compositions. However, certain applications require a polishing composition having a pH within the range at which conventional polishing compositions comprising metal oxide are colloidally unstable. Accordingly, several attempts have been made to provide polishing compositions which contain metal oxide abrasives and exhibit colloidal stability over a broad range of pH values. These attempts have variably involved the addition of additives to the polishing composition or the application of a coating to the surface of the metal oxide particles. However, these additives and coatings can significantly affect the electrical charge present at the surface of the metal oxide particles, which can negatively impact the effectiveness of the polishing composition.
A need therefore exists for a polishing composition comprising metal oxide particles that is colloidally stable over a wide pH range. A need also exists for a method of polishing a substrate with such a polishing composition. The invention provides such a composition and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.